A priming coating is a preparatory coating that is applied to a surface before a subsequent coating is applied. Priming coatings are used in a broad range of applications, inter alia, to improve the adhesion between the subsequent coating and the surface, to increase the durability of the subsequent coating, and to provide additional protection for the surface being coated.
The use of polydopamine as a primer has attracted great interest since the discovery that simple immersion of a substrate in a dilute aqueous solution of dopamine, buffered to alkaline pH, results in the spontaneous deposition of a polydopamine film on the substrate. Messersmith et al., (Science, 2007, 318, 426-430) demonstrated that a polydopamine coating is able to form on a variety of substrate surfaces, including metals, metal oxides, ceramics, synthetic polymers and a wide range of other hydrophilic and hydrophobic materials. Polydopamine coatings have been used as a platform for the conjugation of synthetic polymers or biomolecules to a surface, as illustrated in WO2011/005258 which discloses the attachment of amine-functionalised polyethylene glycol (“PEG-NH2”) to a polydopamine coating, to provide a hydrophilic outer layer for the prevention of biofilm formation. US2008/0149566 discloses that a substrate treated with a surface-modifying agent (SMA) such as polydopamine can be treated with a secondary reactive moiety to impart specific functionalities to the substrate. The secondary moiety is described as an “ad-layer” and may be applied by various means, including by nucleophilic addition and by free radical graft polymerisation.
However, the use of polydopamine as a coating such as a priming coating has certain drawbacks. Polydopamine is known to degrade under oxidative conditions and is also susceptible to degradation under sterilising conditions, thereby reducing its utility for coating medical devices. Although the exact nature of the interaction between a polydopamine coating and the surface which it coats is unknown and is likely to be surface dependent, it is acknowledged the polydopamine layer is not covalently bound to the surface. This has implications for the durability of any subsequent coating that is applied to the polydopamine layer. Furthermore, it has been observed that when a substrate is dipped in a solution of polydopamine, over time particulates of polydopamine are observed in the solution. Particulation is undesirable in most coating applications that require a long-term or permanent, stable coating.
In summary, there remains a need for improved coatings for surfaces, particularly priming coatings. Preferably, such coatings are durable, sterilizable, low particulating, biocompatible and readily applied to a surface.